This invention relates to a liquid jetting apparatus such as an ink jet recording apparatus having a recording head capable of ejecting ink droplets as piezoelectric vibrators operate, and a method of driving such a liquid jetting apparatus.
An ink jet recording apparatus of a printer, a plotter, a facsimile, etc., ejects ink droplets from a recording head and hitting the ink droplets on a print record medium such as recording paper, a print film, or a CD-R (compact disc recordable), thereby recording dots. The recording head used with the recording apparatus may comprise piezoelectric vibrators (PZT) as pressure generating sources. With the recording head, a pressure chamber is expanded or contract as the piezoelectric vibrator becomes deformed, thereby causing pressure fluctuation to occur in ink in the pressure chamber. The pressure fluctuation of ink is used to eject an ink droplet through a nozzle orifice.
The piezoelectric vibrator has the deformation amount determined in response to the supplied voltage value and also has good responsiveness of deformation to voltage change. Thus, the waveform of an ejection pulse signal for ejecting an ink droplet is set appropriately, whereby the ink pressure can be controlled with high accuracy and an ink droplet of any desired amount can be ejected at any desired speed.
To meet the demands for high quality of a record image, increasing the recording speed, etc., a bias voltage is supplied to the piezoelectric vibrator in the normal state and the piezoelectric vibrator is adjusted to a drive potential.
The purpose of adjusting the piezoelectric vibrator to the drive potential is to hold the pressure chamber in an intermediate volume for enabling the volume to be changed to expansion or contraction.
To eject an ink droplet of an extremely small amount at a high frequency, the initial and termination potential of the ejection pulse signal is also set to the maximum potential in the drive signal. In this case, the bias voltage corresponding to the maximum potential is supplied to the piezoelectric vibrator.
By the way, it is known that if a high load is imposed on the piezoelectric vibrator, the lifetime of the piezoelectric vibrator is shortened. Therefore, in the configuration in which a bias voltage is supplied in the normal state, the bias voltage continues to be supplied to the piezoelectric vibrator over a long time. However, preferably the bias voltage is set low as much as possible from the viewpoint of protection of the piezoelectric vibrator.
It is therefore an object of the invention to provide a liquid jetting apparatus suitable for protecting piezoelectric vibrators, and a method of driving such a liquid jetting apparatus.
In order to achieve the above object, according to the present invention, there is provided a liquid jetting apparatus, comprising:
a liquid jetting head, provided with a pressure chamber, a piezoelectric vibrator which causes pressure fluctuation to the pressure chamber and a nozzle orifice communicated with the pressure chamber;
a drive signal generator, which generates a drive signal including a base potential, an initial and termination potential which is a drive potential higher than the base potential, and at least one ejection pulse signal for ejecting a liquid droplet from the nozzle orifice, the drive signal generating the drive signal every recording period;
a drive signal supplier, which selectively supplies the ejection pulse signal to the piezoelectric vibrator in accordance with jetting data which indicates whether a liquid jetting is performed;
a jetting data storage, which stores the jetting data with regard to each of successive two jetting periods including a present jetting period; and
a vibrator potential adjuster, which changes a potential of the piezoelectric vibrator to the base potential when the jetting data stored in the jetting data storage indicates that the liquid jetting is not performed in a latter jetting period, and changes the potential of the piezoelectric vibrator to the drive potential before the ejection pulse is supplied when the jetting data indicates that the liquid jetting is performed in the latter jetting period.
In the apparatus, the vibrator potential adjuster adjusts the vibrator potential in response to combination of record and non-jetting in the former jetting period and the latter jetting period. For example, if a jetting state is indicated in the former jetting period and a non-jetting state is indicated in the latter jetting period, the vibrator potential just after the ejection pulse signal is supplied is the drive potential and thus then the vibrator potential is dropped from the drive potential to the base potential. On the other hand, if a non-jetting state is indicated in the former jetting period and a jetting state is indicated in the latter jetting period, the vibrator potential is the base potential and thus is raised from the base potential to the drive potential before the ejection pulse signal is supplied. Further, the jetting state remains unchanged in the former jetting period and the latter jetting period, the vibrator potential is not adjusted.
Thus, if liquid jetting is not conducted in the latter jetting period, the vibrator potential is adjusted to the base potential. Since the base potential is a low potential fitted for protecting the piezoelectric vibrator, if a non-jetting state continues and the piezoelectric vibrator is maintained at the base potential over a long time, the load imposed on the piezoelectric vibrator is reduced. Therefore, the piezoelectric vibrator can be protected.
If a non-jetting state is indicated in the former jetting period and a jetting state is indicated in the latter jetting period, the vibrator potential is raised from the base potential to the drive potential. Since the drive potential is also the leading end potential of the ejection pulse signal, when supplying the ejection pulse signal is started, the potentials of the vibrator potential and the ejection pulse signal can be matched with each other and the ejection pulse signal can be supplied smoothly to the piezoelectric vibrator. Thus, the load imposed on the piezoelectric vibrator can be reduced and the piezoelectric vibrator can be protected.
Accordingly, if the piezoelectric vibrator is driven at a high frequency by the ejection pulse signal having the high initial and termination potential, the load imposed on the piezoelectric vibrator can be reduced and the piezoelectric vibrator can be protected. Further, raising and dropping the vibrator potential in a short time is decreased, so that the ink pressure in the pressure chamber is easily stabilized and the deflected flight of an ink droplet can also be prevented.
Preferably, the jetting data is binary data which is associated with whether the liquid jetting is performed. The jetting data storage stores jetting data with regard to the present jetting period and a next jetting period, so that a potential of the piezoelectric vibrator when the present jetting period is terminated is changed to the base potential or the drive potential.
Preferably, the jetting data includes: gradation data which indicates a gradation of an ink dot recording in the present jetting period; and history data which indicates whether an ink dot recording was performed in a previous jetting period.
Preferably, the vibrator potential adjuster includes a resistance element and a switch which connects the piezoelectric vibrator to either a source of the base potential or a source of the drive potential, via the resistance element.
Preferably, a first dummy data indicating that the liquid jetting is not performed is provided before a first data of jetting data associated with one main scanning of the liquid jetting head, and a second dummy data indicating that the liquid jetting is not performed is provided after a last data of the jetting data.
Preferably, the drive signal includes: a first joint pulse signal which raises the potential of the piezoelectric vibrator from the base potential to the drive potential; and a second joint pulse signal which drops the potential of the piezoelectric vibrator from the drive potential to the base potential. The vibrator potential adjuster supplies either the first joint pulse signal or the second joint pulse signal.
Here, it is preferable that the drive signal generator generates the first joint pulse signal before the ejection pulse signal, and generates the second joint pulse signal after the ejection pulse signal.
Alternatively, the drive signal generator may generate the first joint pulse signal and the second joint signal before the ejection pulse signal.
Further, it is preferable that at least one of the first joint pulse signal and the second joint pulse signal constitutes a part of the ejection pulse signal.
Still further, it is preferable that the drive signal includes a vibrating pulse signal which vibrates a meniscus of liquid in the nozzle orifice such an extent that a liquid drop is not ejected from the nozzle orifice. At least one of the first joint pulse signal and the second joint pulse signal constitutes a part of the vibrating pulse signal.
Still further, it is preferable that a time period for which the potential of the piezoelectric vibrator is varied by the first joint pulse signal and the second joint pulse signal is substantially identical with a natural period of ink in the pressure chamber.
According to the present invention, there is also provided a method of driving a liquid jetting apparatus which comprises a liquid jetting head provided with a pressure chamber, a piezoelectric vibrator which causes pressure fluctuation to the pressure chamber and a nozzle orifice communicated with the pressure chamber, the method comprising the steps of:
generating a drive signal every jetting period, the drive signal including a base potential, an initial and termination potential which is a drive potential higher than the base potential, and at least one ejection pulse signal for ejecting a liquid droplet from the nozzle orifice;
storing recording data which indicates whether a liquid jetting is performed, with regard to each of successive two jetting periods including a present jetting period;
changing a potential of the piezoelectric vibrator to the base potential when the jetting data stored in the jetting data storage indicates that the liquid jetting is not performed in a latter jetting period;
changing the potential of the piezoelectric vibrator to the drive potential before the ejection pulse is supplied when the jetting data indicates that the liquid jetting is performed in the latter jetting period; and
supplying selectively the ejection pulse signal to the piezoelectric vibrator in accordance with the jetting data.
Preferably, the jetting data is binary data which is associated with whether the liquid jetting is performed. The jetting data storage stores jetting data with regard to the present jetting period and a next jetting period, so that a potential of the piezoelectric vibrator when the present jetting period is terminated is changed to the base potential or the drive potential.
Preferably, the jetting data includes: gradation data which indicates a gradation of an ink dot recording in the present jetting period; and history data which indicates whether an ink dot recording was performed in a previous jetting period.
Preferably, the drive signal includes: a first joint pulse signal which raises the potential of the piezoelectric vibrator from the base potential to the drive potential; and a second joint pulse signal which drops the potential of the piezoelectric vibrator from the drive potential to the base potential. The vibrator potential adjuster supplies either the first joint pulse signal or the second joint pulse signal.
Preferably, the piezoelectric vibrator is connected to either a source of the base potential or a source of the drive potential via a resistance element to adjust the potential of the piezoelectric vibrator.